1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording information densely on an optically recording thin film formed on a substrate with an optical or thermal energy and optically reproducing the information.
2. Description of Related Art
It is possible to make a small laser spot having a diameter of the order of the wave length of the laser when the laser is focused by a lens system. Therefore a laser source in a low output power can make a laser beam in a high energy density per unit area. The laser beam consequently can change a material state at a small area and also can read the changed state at the small area of a material. An optical information recording medium can use the above change by the laser beam and to record and reproduce the information. Hereinafter the optical information recording medium is referred to as an optical recording medium or simply a medium.
The optical recording medium in a basic structure is characterized by a substrate provided with a flat surface having a recording film applied thereto. The recording film should vary in the state, i.e. exhibit some optically detectable change, upon irradiating the laser beam. The following method usually executes the recording and reproducing of the information by using the optical recording medium. The optical recording medium is moved rotatably or straightly by a suitable means, for example, a motor, and is irradiated by a convergent laser beam. During the above process the concentrated laser beam should be usually controlled to focus on the recording surface. The recording surface is heated by absorbing the laser energy. When the laser is in a power higher than a predetermined laser power threshold value, the recording film changes in the state and records information. The threshold value depends on the characteristic of the recording film, the thermal characteristic of the substrate and a moving speed of the optical recording medium relative to the laser beam spot. When a laser beam spot having an output power lower than the threshold value is irradiated on the recording film, a part having the recorded information exhibits optical characteristics such as the intensity of a transmitted light, the intensity of a reflected light and their polarization angles different from those of a part having no recorded information. These differences make it possible to read and reproduce the recorded information. During a reading process or a reproducing process, the laser beam spot is usually subjected to a tracking control so as to follow exactly a line of the parts changed in the state by the recorded information prepared by an irradiation of laser. For an execution of the tracking control, the substrate has a tracking guide such as an uneven groove formed thereon. It has long been known to execute the recording and reproducing of information on the optical recording medium under such a tracking guide.
An example of the optical recording medium is an optical recording disk. The optical recording disk is already used in the market as a video picture file, a documentation file and a data file of a computer. An optical recording medium in a card form or a tape form is proposed for a practical use.
It is desirable to achieve, as a recording film, a material or a structure which shows a large change in the optical characteristic under irradiation of a laser of a small power.
It has been known that the optical recording medium can be composed of a metal film made from Bi or Te metal film, an alloy film including Bi or Te as a main constituent or a compound film including Te. The film has holes made by melting or evaporation to record information upon receiving a laser beam. The reflected beams or the transmitted beams at the part having holes and the part having no holes are different from each other in the phase and are canceled or diffracted by the interference between them and differ consequently in the amount of the reflected beam or the transmitted beam. The optical recording medium in such characteristic can be composed of Se-Te system (disclosed in Japanese Patent Publication (unexamined) 59-35356) and Te-C system (disclosed in Japanese Patent Publication (examined) 58-71195) or possibly an organic pigment.
In addition, there has been known another optical recording medium called a phase transition type which shows an optical change due to a change in the crystal structure without changing the shape. A phase transition in the crystal structure has been known to have two types, one is a phase transition between an amorphous state and a crystal state, the other, a phase transition between two crystal states. As to the optical recording medium of the phase transition type, there have been known thin films of amorphous chalcogen compound and oxide containing Te-TeO.sub.2 as a main component (see, for example, Japanese Patent Publication (examined) No. 54-3725 and U.S. Pat. No. 3,971,874). Also, thin film containing Te-TeO.sub.2 -Pd as a main component (see, for example, Japanese Patent Publication (unexamined) No. 61-68296 and U.S. Pat. No. 4,624,914) has been known. The signal recording is carried out by changing either the extinction coefficient or the refractive index of the thin film by irradiating the same with a laser beam. The signal reproduction can be carried out by detecting a variation in the amount of transmitted laser or reflected laser which is caused by a variation in the amplitude of transmitted laser or reflected laser at the part having the information recorded. Further the disclosure has reported an optical recording medium having two stacked layers of a layer absorbing a laser and a layer changing an optical characteristic (Japanese Patent Publication (examined) 1-14039).
There is known another type of an optical recording medium which is capable of only reproducing information signals. The read only medium optically reproduces information recorded on a replica disk made from a resin material, which has information signals recorded in a form of uneven pits previously formed thereon. The replica disk is manufactured in such a way that the resin substrate is stamped with a stamper made of metal such as nickel to form the uneven pits and is provided with a laser reflecting layer of aluminum (Al) or gold (Au) applied to the surface of the resin disk by a vacuum evaporation method. This method is capable of manufacturing the resin disks in a large scale. The read only medium is characterized by a high reflectivity due to the reflecting layer and a high level of a reproducing signal. The high reflectivity would be able to make it easy to treat the read only medium with a focus tracking control. The optical read only medium such as an optical video disk (VLP), an audio digital disk (CD: compact disk) or a read only memory (ROM) of a computer which is called CD-ROM is now commercially available.
A disclosure has reported a read and write recording medium compatible with the reproducing only medium. There is a disclosure (Japanese Patent Publication (examined) 2-132656) which reports a recording medium which comprises a resin substrate having an organic pigment layer and a metal reflecting layer formed thereon to achieve a high reflectivity. An information signal can be recorded on the recording medium and reproduced by an equipment designed for a read only medium.
Among the optical recording mediums, an optical recording medium of ablative type can produce the reflected laser of a high level but has a disadvantage that the pit can not be easily formed in an accurate shape and size, and the noise generated during the reproducing process is in a rather high level. In addition, it is difficult to apply a contact protecting layer to the recording medium of ablative type. The recording medium of ablative type must have such a complicated structure which is called air-sandwich structure which has an air gap between the protecting layer and the recording medium, and results in a difficulty in the manufacturing and accordingly in a high cost.
An optical recording medium of phase transition type does not show a change in the shape and consequently is simple in the structure thereof resulting in ease and a low cost in the manufacturing process. However, the optical recording medium of phase transition type has small optical constants and consequently a low reflectance. In order to achieve a high reflectance of the optical recording medium of phase transition type, it is necessary to form a complicated multi-layered structure designated according to an optical design technique and an additional metal reflecting layer.
In connection with a conventional optical recording medium, it can be said that the optical recording medium having a high reflectance exhibits a poor laser absorption and consequently a poor recording sensitivity. Particularly the write once type of optical recording medium accompanied with a metal reflection film, and the optical recording medium of phase transition type include a recording film having a high thermal conductivity and require a high power of the laser for recording information. The requirement of the high laser power prevents these optical recording mediums from a practical application.
An optical recording medium comprising an organic dye has a high recording sensitivity because of its low threshold temperature of a transition and a low thermal conductivity, and consequently can record information with a practical power size of laser in spite of a low absorption of laser. On the other hand, the optical recording medium comprising an organic pigment has a small optical constant and consequently a low reflectance. In order to improve the reflectance, an additional metal reflecting layer is required and results in a complicated structure of the optical recording medium. Further there is another problem that the optical recording medium comprising an organic pigment has a low aging stability and particularly a low resistance to a degradation caused by an irradiation of ultraviolet ray.